This invention relates to spark plugs and more particularly to spark plugs having improved center electrodes.
Smooth operation of an internal combustion engine is directly related to the condition of the spark plugs used in the engine. Such spark plugs are subjected to intense heat, in a typical four cycle engine the flame temperatures are over 3000.degree. F. as well as the extremely high pressures used to drive the pistons. Additionally, the voltage pulse used to ignite the fuel can be in excess of 30,000 volts at times. Not only must the spark plug withstand these conditions for many thousands of miles but the heat transfer characteristics must be such over the life of the plug that heat will be dissipated evenly from the firing tip to avoid preignition with possible engine damage resulting therefrom.
In general the heat path extending from the tip of the insulator body mounting the center electrode to its point of contact with the shell mounting the spark plug to the cylinder head determines the operating temperature of the spark plug. Heat is transferred to the cylinder head to prevent excessive temperatures which would result in preignition and detonation. On the other hand the operating temperature of the plug must be maintained sufficiently high to prevent carbonizing and eventual short circuiting of the gap.
Thus the length of the heat path as well as the thermal conductivity of the components making up the heat path must be taken into account in designing the spark plug. In general, the longer the heat path, the higher the operating temperature of the plug. Another primary factor to be considered in the plug design is the effects of spark erosion and corrosion. In choosing the material to be used for the center electrode various criteria relating to performance are considered including resistance to corrosion and erosion, electrical resistivity and thermal conductivity as well as criteria relating to manufacturing such as formability, weldability, cost and availability of the raw material. For many years it was conventional to use various special steel alloys for the center electrode however as corrosion became more and more of a problem in automotive engines, these alloys were supplanted by nickel and nickel chrome alloys which are still in wide use today. Nickel material has offered the best compromise for the above criteria for an electrode having a single layer of material; however, such material is more costly than the older steel alloys and the thermal conductivity is not as high as would be desirable. Among the various attempts which have been made to increase the thermal conductivity of the center electrode include the use of high conductivity material, such as copper encased within the nickel material so that the surface of the material has the desirable corrosion and erosion resistance characteristics while the copper enhances heat transfer. Examples of the approach are disclosed in U.S. Pat. Nos. 2,955,222; 3,144,576; 3,356,882; 3,803,892 and 3,857,145. However, this approach has not been widely accepted since such electrodes are relatively expensive to manufacture and the heat transfer characteristics is still not optimum. In higher cost aircraft spark plugs, which are subject to more severe service conditions than most other internal combustion engine applications and in which a higher degree of reliability is desirable it is known to provide a separate sleeve of copper surrounding the center electrode to act as a heat sink. While this construction improves the heat transfer characteristics of the spark plug it is relatively expensive to manufacture since the dimensional tolerances of the diameters of the electrode sleeve and insulator must be held very close to obtain optimum heat transfer therethrough and additional manufacturing steps are required to assemble the parts in fixed relation within the plug.
Accordingly, an objective of the invention is the provision of an internal combustion engine spark plug center electrode which has improved heat transfer characteristics while maintaining acceptable corrosion and erosion characteristics.
Another object is the provision of such an electrode which is inexpensive and readily compatible with current manufacturing techniques.